1. Field of the Invention
This invention relates to a microcomputer and a non-contact IC card using the same.
2. Description of the Related Art
The structure of a conventional non-contact IC card is shown in FIG. 6. A CPU 1 is connected with a ROM 2a, a RAM 2b, a transmission circuit 3, and a reception circuit 4, all contained within a microcomputer 5. The transmission circuit 3 and the reception circuit 4 of the microcomputer 5 are connected with a data transmission antenna 6 and a data reception antenna 7, respectively. The CPU 1 is connected with a battery 8 and an oscillator 9. The whole IC card is sealed with resin or the like in order to improve resistance to the environment.
A power supply voltage is supplied to the CPU 1 from the battery 8, and a clock signal is supplied to the CPU 1 from the oscillator 9, causing the CPU 1 to be actuated on the basis of a program stored in the ROM 2a. The IC card sends and receives data to and from the outside using electromagnetic waves. At the time of data reception, electromagnetic waves from the outside are received by the reception antenna 7 and decoded to data by the reception circuit 4, after which they are input to the CPU 1. Data processing is performed in the CPU 1, and data is stored in the RAM 2b when needed. On the other hand, at the time of data transmission, data from the CPU 1 is output to the transmission circuit 3. At this point, carrier waves are coded with this data and sent from the transmission antenna 6.
An example of the configuration of the transmission circuit 3 is shown in FIG. 7. A capacitor 11 is connected in parallel with the transmission antenna 6, and these elements constitute a resonant circuit 12. The resonant circuit 12 is connected with the output terminal of the gate circuit 15 via a resistor 13 and a transistor 14. One input terminal of the gate circuit 15 is connected with the CPU 1, and a transmission signal indicating data and carrier waves are input to another terminal of the gate circuit 15 from the CPU 1.
In the transmission circuit 3, shown in FIG. 8, when a transmission signal of "H" level is output to the gate circuit 15 from the CPU 1 at time t.sub.1, the transistor 14 is turned on by the carrier wave. Then, the resonant circuit 12 is activated and electromagnetic waves are launched from the transmission antenna 6. On the other hand, when a transmission signal is at "L" level, as at time t.sub.0, the transistor 14 is off. Therefore, the resonant circuit 12 is not activated and electromagnetic waves are not sent. In this way, electromagnetic waves are modulated by a transmission signal and transmitted.
However, as shown in FIG. 8, although when the level of the transmission signal changes from "H" to "L" at time t.sub.2, the transistor 14 goes off instantly, the oscillations between time t.sub.1 to t.sub.2 in the resonant circuit 12 only gradually decay. Therefore, a long period of time .DELTA.t is required from the time the level of a transmission signal becomes "L" to the time the oscillation in the resonant circuit 12, namely, the electromagnetic waves sent from the transmission antenna 6, disappear. To accurately transmit data, a pulse must be transmitted only after the oscillations of the previous pulse of a transmission signal decrease to negligible magnitude. Because of this requirement, the speed of data transmission is slow in the prior art.